1. Field of the Invention
The present invention relates to a method and a device for supplying fuel to an internal combustion engine. More specifically, the invention relates to a method and a device in which a liquid fuel supplied to the engine is in a supercritical state.
2. Description of the Related Art
In general, a fuel supply system for an internal combustion engine is required to supply liquid fuel to combustion chambers of the engine in the form of a very fine mist. Especially, in a direct cylinder injection system for injecting fuel into the combustion chamber directly, such as the injection system of a diesel engine, the size of particles of the injected fuel largely affects the performance of the engine. In the direct cylinder injection system, the combustion of the fuel in the combustion chamber, and thus the performance of the engine can be largely improved by reducing the size of the particles of the injected fuel.
It is thought that high pressure fuel injection, in which fuel is injected from the fuel injection valve at a very high pressure, is effective in order to reduce the size of the particles of the injected fuel. Therefore, in recent fuel injection systems, the fuel injection pressure is set at a very high value in order to satisfy the requirement for low exhaust gas emission and low fuel consumption. For example, some of the fuel injection systems for diesel engines use a fuel injection pressure as high as more than 50 MPa (500 atm) to generate very fine particles of injected fuel in the combustion chamber.
However, in the high pressure fuel injection system a fuel pump which is capable of pressurizing the fuel to a very high pressure is required. Since the high pressure fuel pump is very expensive, this increases the cost of whole fuel injection system. Further, in the high pressure fuel pump, the clearances between the sliding parts must be small in order to prevent the high pressure fuel from leaking. This tends to cause wear of the sliding parts and lowers the reliability of the pump.